Field of the Invention
The present invention relates to a position correction control that corrects a position of an image formed on an image bearing member in an image forming apparatus.
Description of the Related Art
An image forming apparatus of an electrophotographic system has image forming units that form images using toners for respective color components. The images formed with these image forming units are transferred onto an image bearing member so as to superimpose. As a result of this, a multicolor image is generated. The image forming apparatus transfers the multicolor image on the image bearing member to a sheet, fixes the multicolor image to the sheet with heat and pressure by a fixing device, and outputs the printed sheet.
Since such an image forming apparatus superimposes images formed with a plurality of image forming units, when at least one image forming unit forms an image at a position different from a target position, color misregistration occurs in a multicolor image on a printed sheet, which lowers quality of the multicolor image.
Accordingly, an image forming apparatus makes an image forming unit form a measurement image with a toner in a predetermined color, measures the measurement image with a sensor, and adjusts an image forming position of the image forming unit on the basis of a measurement result of the sensor. As a result of this, the color misregistration of the multicolor image is reduced.
The sensor that measures the measurement image is provided with a light emitting element and light receiving element, for example. The light emitting element irradiates the image bearing member, and the light receiving element receives reflected light from the image bearing member and reflected light from the measurement image. An output value of the sensor varies according to intensity of the reflected light from the measurement image received with the light receiving element. The image forming apparatus determines positional relationship of the measurement image on the basis of the output value of the sensor, and corrects relative misregistration of the image forming position on the basis of the positional relationship concerned. However, when difference between a reflectance of the image bearing member and a reflectance of the toner of the predetermined color is minute, the positional relationship of the measurement image may not be determined. That is, when the difference between the intensity of the reflected light from the measurement image and the intensity of the reflected light from the image bearing member is minute, the image forming apparatus may not distinguish the reflected light from the measurement image and the reflected light from the image bearing member.
The technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2012-3234 (JP 2012-3234A) measures a position of a measurement image formed with a toner of a predetermined color using a superimposed measurement image. The superimposed measurement image is formed by superimposing a measurement image that is formed using the toner of the predetermined color on a measurement image formed using a toner of another color different from the predetermined color. It should be noted that the reflectance of the toner of the other color differs from the reflectance of the image bearing member. In the superimposed measurement image of the above-mentioned publication, the measurement image of the predetermined color has a slit and the measurement image of the other color appears in the slit. The above-mentioned sensor outputs the output value corresponding to the intensity of the reflected light from the measurement image of the other color appeared in the slit. Since the output value of the sensor also varies when the positional relationship between the measurement image of the predetermined color and the measurement image of the other color varies, the image forming apparatus is able to measure the position of the measurement image in the predetermined color.
However, when the misregistration of the measurement image of the predetermined color goes beyond a tolerance, the measurement image of the predetermined color may be superimposed on another measurement image different from the superimposed measurement image. Accordingly, when the misregistration of the measurement image of the predetermined color goes beyond the tolerance, the image forming position of the measurement image of the predetermined color is misdetected. Accordingly, when the misregistration of the measurement image of the predetermined color goes beyond the tolerance, the image forming apparatus cannot correct the color misregistration appropriately.